DE69502601T2 - METHOD FOR PRODUCING ELECTRICAL HEATING RESISTORS - Google Patents

Abstract

An electrical resistance heating element is made from an electrical resistance material having the following composition in weight percent: - Group A: - aluminium 3-8 - yttrium, zirconium, hafnium and/or 0-0.45 - one or more rare earth elements - Group B: - chromium 12-30 - iron and/or nickel and/or cobalt balance - The electrical resistance material is arranged in an atmosphere having a potential for oxidation such as to permit oxidation of the constituent(s) from Group A and to inhibit oxidation of the constituents from Group B. The resistance material is then heated in the atmosphere to a temperature in the range from 800 DEG C. to a temperature below its melting point so as to oxidize the constituent(s) of Group A at the surface and to form a surface layer consisting essentially of continuous unified oxide of the constituent(s) of Group A.

Description

This invention relates to a method for manufacturing an electric resistance heating means, for example for use as a heating element in an electric radiant heating device as used in a smooth-topped cooker with a glass-ceramic plate.

It is well known to make use of alloys in which the main constituents are chromium and aluminium in combination with iron and/or nickel and/or cobalt for the manufacture of heating elements for electric cooker radiant heaters. When connected to a power source, the heating elements are automatically heated electrically to radiant brightness at operating temperatures which may be from about 900°C to 1150°C. During such high temperature operation in air, the aluminium present in the alloy forms a protective layer of aluminium oxide on the surface of the alloy with a thickness of, for example, about 5 to 15 µm. Other constituents of the alloy such as iron and chromium which are present on the free surface of the alloy are also oxidised and it may therefore happen that the aluminium oxide does not form a continuous or uninterrupted layer covering the entire surface of the alloy. In addition, under these conditions, mixed aluminium oxide phases are formed on the surface of the alloy, for example a mixture of alpha, beta, gamma and other transition phases of aluminium oxide and certain other phases of substances that are not aluminium oxide. The surface layer formed is considerably permeable to atmospheric oxygen and therefore does not offer perfect protection. If the alloy is therefore continuously exposed to high temperatures as a heating element, penetrating atmospheric oxygen continuously oxidises aluminium that has diffused from the interior of the alloy to the surface. A layer consisting mainly of aluminium oxide forms on the surface of the alloy. existing layer of increasing thickness, while the aluminium content inside the alloy gradually decreases.

The lifetime of a heating element depends largely on the oxidation rate and therefore on the growth rate of the protective aluminum oxide layer. Reduced permeability of the oxide layer would therefore result in increased lifetime of the element.

If oxides of substances other than aluminum are present on the surface of the alloy, for example iron oxide or chromium oxide, this affects the perfect condition of the aluminum oxide layer and can, especially under conditions of thermal cycling caused by repeated heating and cooling of the alloy, as regularly occurs in heating element applications, lead to deterioration and local flaking of the aluminum oxide, so that the protection of the areas of the alloy that are exposed as a result is reduced. The flaking can be reduced by adding small percentage amounts, i.e. amounts within the range of 0.01 to 0.45 wt.%, of active elements such as yttrium or rare earth elements, which change the crystal structure of the aluminum oxide from platelet-shaped to prism-shaped.

After all the aluminum in the alloy heating element has been oxidized, the other components of the alloy, such as iron and chromium, are oxidized until the heating element finally fails. When it comes to relatively thick wire heating elements, for example with a thickness of 800 µm or more, a reasonably long life of, for example, more than 5000 hours can be achieved without any problems. For thin heating elements, such as heating elements made of thin However, problems have arisen with wire and particularly with heating elements in the form of a thin alloy strip or tape, which may, for example, be placed on edge on an insulating base. Such a strip or tape may, for example, be 20 to 200 µm thick. When such a strip or tape is used as a radiant heating element, a thick layer of aluminium oxide is formed on its surface, as described above. Because the strip or tape is thin, it takes less time for all the aluminium to diffuse to the surface than with thicker alloy elements. In addition, the thermal expansion coefficient of the aluminium oxide layer differs considerably from the thermal expansion coefficient of the underlying alloy material, and the thickness of the oxide layer may constitute a significant proportion of the total thickness of the strip or tape. Therefore, in the course of thermal cycling, such as occurs when the heating element is in operation, mechanical stresses occur which lead to increasing permanent deformation of the strip or tape. This means that in certain areas of the strip or tape the thickness of the electrically conductive material is less than in other areas. In such areas of reduced thickness the temperature can rise to a higher value than in the remaining areas of the strip or tape if the strip or tape is electrically connected and acts as a heating element. Eventually the heating element fails in one or more of these areas of reduced thickness, for example as a result of cracking caused by stress corrosion.

One might think that increasing the aluminum content of the alloy would be a possible method for improving the service life of such a heating element. However, as the aluminum content increases, the malleability of the alloy decreases and the machining of the Alloying is becoming increasingly difficult. In view of this problem, the weight fraction of aluminium in the alloy is limited to about 8 percent, although in practice the aluminium content is generally somewhat less than 8 percent.

EP-A-0 327 831 relates to improving the adhesion of an oxide layer to an electrical resistance material comprising aluminium, chromium and iron, nickel or cobalt. The adhesion is improved by a second heating step which causes recrystallization in the surface layer.

US-A-4 414 023 relates to the formation of an adherent structured aluminum oxide surface on an iron-chromium-aluminum alloy for use in an electric heating element. The alloy contains cerium, lanthanum and other rare earths.

It is an object of the present invention to provide a method of manufacturing a heating element made of aluminium alloy which, in use, results in a reduction in the rate at which aluminium is lost from the interior of the alloy and which therefore results in a longer service life of the element.

According to the invention, a method for producing an electrical resistance-based heating means is provided, which comprises the following steps:

Providing an electrical resistance material comprising an alloy composed in group A of the elements aluminium and optionally yttrium, zirconium, hafnium and/or one or more rare earth elements and in group B of the elements chromium and one or more of the elements iron and/or nickel and/or cobalt; and Heat treatment of the electrical resistance material in a heating stage carried out in a housing, the heating stage comprising the following steps:

(a) creating an atmosphere in the housing surrounding the electrical resistance material, the oxidizing potential of the atmosphere being such that oxidation of the group A component(s) is enabled and oxidation of the group B components is inhibited; and

(b) heating the electrical resistance material in the atmosphere in the housing to a temperature between 800°C and a temperature lower than the melting point of the alloy to oxidize the Group A constituent(s) at the surface of the alloy to form a surface layer consisting essentially of continuous, unified oxide of the Group A constituent(s),

where the composition of the alloy in weight percent is as follows:

Group A:

Aluminum 3-8

Yttrium, zirconium, hafnium and/or one or more rare earth elements 0 - 0.45

Group B:

Chrome 12 -30

Iron and/or nickel and/or cobalt residue

and wherein an atmosphere consisting exclusively of water vapor is introduced into the housing so that the heat treatment is carried out in a single step.

We have found that the preparation of a surface oxide layer according to the invention, which in practice consists essentially of alumina, results in a surface layer which is slightly permeable to air and other oxidizing atmospheres and provides considerable resistance to subsequent oxidation of the aluminum in the underlying internal alloy material, so that the thickness of the surface layer increases at an unexpectedly low rate.

The alloy need not contain any active element, in which case the composition of the alloy in weight percent may be as follows:

Group A:

Aluminium 3 - 8 preferably 4.5 - 6

Group B:

Chromium 12 - 30 preferably 19 - 23

Iron and/or nickel and/or cobalt rest

However, if the alloy contains an active element, the composition of the alloy in weight percent may be as follows:

Group A:

Aluminium 3 - 8 preferably 4.5 - 6

Yttrium, zirconium, hafnium 0.01 - 0.45 and/or one or more rare earth elements 0.01 - 0.45 preferably 0.025 -0.4

Group B:

Chromium 12 - 30 preferably 19 - 23

Iron and/or nickel and/or cobalt rest

The one or more rare earth elements may comprise lanthanum and/or cerium, but preferably lanthanum. If the rare earth elements comprise lanthanum and cerium, the total content of these elements in the alloy may be within the range of 0.025 to 0.07 wt.%. The lanthanum content is preferably within the range of 0.005 to 0.02 wt.%, while the cerium content is within the range of 0.02 to 0.05 wt.%. If the rare earth element comprises lanthanum, the lanthanum content of the alloy may be within the range of 0.06 to 0.15 wt.%.

If the active element comprises zirconium, the zirconium content of the alloy may be within the range of 0.1 to 0.4 wt.%.

The thickness of the surface layer should be less than about 2 µm, preferably less than about 1 µm, and ideally about 0.3 to 0.5 µm. By limiting the surface layer to a thickness that is small in relation to the thickness or diameter of the electrical resistance material, stress-induced deformation of the resulting heating medium under thermal cycling is minimized.

The heating may be carried out at a temperature of from 900°C to 1475°C, preferably from 900°C to about 1300°C. The heating may be carried out at a temperature of at least about 1000°C, but preferably at least about 1100°C.

If the alloy contains predominantly lanthanum as the active element, a temperature of about 1200ºC is usually preferred over lower temperatures, but if the alloy contains zirconium, a temperature of about 1300ºC is usually preferred over lower temperatures.

The temperature and the duration of the heating phase can be mutually dependent, i.e. the higher the temperature, the shorter the duration.

The electrical resistance material can be heated in the atmosphere at a temperature of about 1200ºC for about 8 minutes. At a temperature of about 1360ºC to 1400ºC, the duration is about 5 minutes, and at a temperature of about 1450ºC to 1475ºC, the duration is about 2 minutes.

The alumina may be present essentially as alpha alumina. Alpha alumina is the densest form of alumina and its permeability to air and other oxidizing atmospheres is lower than that of the solid solutions of alumina formed according to the prior art.

The electrical resistance material is preferably monolithic, for example rolled or drawn from a cast block, but can also consist of sintered material.

As a result of the manufacturing process, the permeability of the surface layer to air or other oxidizing atmosphere is low.

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of an embodiment of a heating element for an electric radiant heating device;

Figure 2 is a plan view of an electric radiant heating device incorporating the heating element shown in Figure 1;

Fig. 3 is a sectional view of the electric radiant heater device shown in Fig. 2;

Figure 4 is a plan view of an electric radiant heating device comprising a heating element of optional shape;

Fig. 5 is a sectional view of the electric radiant heater shown in Fig. 4; and

Fig. 6 is a diagrammatic representation of an embodiment of a treatment device for carrying out the method according to the invention.

As can be seen in Fig. 1, a heating element 4 intended for use as a heating element in a radiant heating device for a cooker with a glass-ceramic plate is manufactured by corrugating a strip 5 and then bending the said strip into the shape required for the element.

As can be seen in Figures 2 and 3, the heating element 4 is secured to a base 2 of thermally and electrically insulating material, preferably a microporous thermally insulating material, in a metal basin 1. The element 4 is conveniently secured by embedding in the base 2 the strip 5 which forms it up to a part of its height. If desired, the strip 5 of the element 4 be profiled along that edge of the strip which is embedded in the insulating material, for example by providing downwardly extending and spaced-apart integrating tabs (not shown) embedded in the insulating material of the base 2.

A connection 6 is provided for connecting the heating element 4 to a power source for its operation.

Adjacent to the side of the basin 1 is a peripheral wall 3 made of thermally insulating material, the upper surface of which is arranged so that, during use, it is in contact with the underside of a glass-ceramic hotplate of a cooker.

A heat cut-out device 7 of well-known form is provided extending across the heating element 4 and functioning to cut off the heating element to prevent excessive heating when the heating device is installed in a cooking oven and in operation.

The strip 5 forming the heating element 4 has, for example, a height h of 1.5 to 6 mm and a thickness of 20 to 200 µm.

In Figures 4 and 5, the corrugated strip shape of the heating element shown in Figure 1 is replaced by a spirally wound coil-shaped heating element 14 which is fixed to the base 2 located in the metal basin 1. The heating element 14 is fixed in grooves 15 formed in the base 2 by any convenient means, for example by means of metal clips (not shown).

For the purpose of operating the heating element 14, the connection 6 enables electrical connection of the heating element to a power source.

The peripheral wall 3 rests against the side of the basin 1, and in use the upper surface of the peripheral wall is in contact with the underside of the glass-ceramic cooking plate 16 of a cooker.

The heat cut-out device 7 extends across the heating element 14 and has the function of switching off said heating element in order to prevent excessive heating when the heating device is installed in a cooking oven and is in operation.

The wire forming the heating element 14 may have any suitable diameter, for example between 250 and 750 µm or more.

EXAMPLE

An embodiment of the invention will now be described by way of example.

As shown in Fig. 6, a sample 18 of a resistive element in the form of a thin strip or ribbon having a thickness of about 50 µm was placed at about room temperature within a treatment housing 20. The treatment housing 20 was designed as a quartz tube with metal end caps 22, said caps being provided on their inner surface with a layer of thermally insulating material. Outside the treatment housing was arranged a thermocouple 24 which served to determine the surface temperature of the housing.

The resistance element was made of an alloy with the following composition in weight percent:

Aluminum 4.5 - 6

Lanthanum 0.06 - 0.15

Chrome 19 - 22

Iron Rest

Electrical current supply wires 26 were connected to the ends of the sample 18 and passed through the end caps 22 for connection to a voltage source V.

Water vapor supplied by a steam generator (steam generator) of known form 28 was introduced into the treatment housing 20 through a quartz tube 30 to thoroughly displace the air present within the housing with water vapor and to maintain the housing in a water vapor-filled condition. The displacement of the air by the water vapor was demonstrated by passing the outgoing air along a quartz tube 32 and into a water bath 34 so that bubbles indicated the outgoing of air from the treatment housing. The formation of air bubbles ceased once all the air was displaced. An electrical heating element in the form of an electrical heating tape 36 was wound around the quartz tube 30 and the outside of the housing 20, the purpose of which was to heat the tube 30 and the housing 20 to about 200°C, thereby minimizing the risk of water vapor condensation.

Sample 18 was then electrically heated in the steam atmosphere to a temperature of about 1200ºC for 8 minutes by passing an electric current through the sample at a temperature such that partial decomposition of the steam into the metals used to oxidize the aluminium and the lanthanum. oxygen and hydrogen to inhibit oxidation of the chromium and iron. During the treatment, it was ensured that the housing was pressurized with steam and this pressure was evident by a visible stream of steam emerging from a small hole 37 at the end of the housing 20. After treatment, the sample was removed from the housing and found to have formed on its surface a substantially continuous, uniform, dense and thin aluminum oxide layer consisting of aluminum contained in the alloy material forming the strip, but oxidation of iron or chromium present on the surface of the strip alloy material was inhibited.

In relation to the thickness of the strip, the thickness of the aluminum oxide layer formed was small, for example about 0.5 µm, and the layer adhered firmly to the underlying alloy material of the strip.

The treatment was repeated on four other samples of alloy material of the same composition, but the duration and temperature were varied in accordance with Table 1 below: Table 1

The treated samples were tested as well as a control sample made from the same strip material that had not been treated in the manner described above.

All samples were connected to an electrical power source and heated automatically and periodically with an electrical current between approximately room temperature and 1150ºC in air until they failed. It was found that the treated samples withstood the thermal cycling test for approximately twice as long as the untreated control sample before they failed.

The heating element does not need to be in the form of a strip or ribbon and can optionally consist of wire with a diameter between, for example, 250 and 750 µm or more.

The method described in the example was carried out in combination with alloys of other compositions. The composition in weight percent of an alloy used is as follows:

Aluminum 5 - 6

Zircon 0.1 - 0.4

Chrome 21 - 23

Iron Rest

The composition in weight percent of another alloy is as follows:

Aluminum 5 - 6

Cerium 0.02 - 0.05

Lanthanum 0.005 - 0.02

Chrome 19 - 21

Iron Rest

We also made use of an alloy with the following composition in weight percent:

Aluminum 4.5 - 5

Chrome 19.5 - 21.5

Iron Rest

In each case, we treated samples of the material in tape and wire form, respectively, for 2 minutes and 8 minutes at temperatures of 1000ºC, 1100ºC, 1200ºC and 1300ºC, in accordance with the method described in the example.

In each case, a thin layer of aluminum oxide formed on the material.

All samples were connected to an electrical power source and automatically cycled by means of an electrical current between about room temperature and 1150ºC. All samples were found to withstand the thermal cycling test better than an untreated control sample because the thickness of the oxide layer increased more slowly. However, with regard to the treated samples, it was found that the samples treated at 1100ºC generally passed the test better than those treated at 1000ºC. When the alloy contained predominantly lanthanum as the active element, the samples treated at 1200ºC passed the test better than those treated at lower temperatures, while in cases where the alloy contained zirconium, the samples treated at 1300ºC passed the test better than those treated at lower temperatures. This was generally the case regardless of whether the samples were treated for 2 minutes or 8 minutes.

A longer treatment time is therefore unnecessary.

Claims (29)

1. A process for manufacturing an electric resistance heating means (4) comprising the following steps: Providing an electrical resistance material (18) comprising an alloy which in group A is composed of the elements aluminium and optionally yttrium, zirconium, hafnium and/or one or more rare earth elements and in group B of the elements chromium and one or more of the elements iron and/or nickel and/or cobalt ; and Heat treatment of the electrical resistance material in a heating stage carried out in a housing (20), the heating stage comprising the following steps: (a) creating an atmosphere in the housing surrounding the electrical resistance material (18), the oxidizing potential of the atmosphere being such that oxidation of the group A constituent(s) is enabled and oxidation of the group B constituents is inhibited; and (b) heating the electrical resistance material (18) in the atmosphere within the housing to a temperature within the range of 800°C and a temperature lower than the melting point of the alloy to oxidize the Group A constituent(s) at the surface of the alloy to form a surface layer consisting essentially of continuous, unified oxide of the Group A constituent(s), characterized in that the composition of the alloy in weight percent is as follows: Group A: Aluminum 3-8 Yttrium, Zircon, Hafnium and/or one or more rare earth elements 0 - 0.45 Group B: Chrome 12 -30 Iron and/or nickel and/or cobalt residue and wherein an atmosphere consisting exclusively of water vapor is introduced into the housing (20) so that the heat treatment is carried out in a single step. 2. A method according to claim 1, characterized in that the composition of the alloy in weight percent is as follows: Group A: Aluminum 3-8 Yttrium, zirconium, hafnium and/or one or more rare earth elements 0.01 - 0.45 Group B: Chrome 12 - 30 Iron and/or nickel and/or cobalt residue 3. A method according to claim 2, characterized in that the composition of the alloy in weight percent is as follows: Group A: Aluminum 4.5 - 6 Yttrium, zirconium, hafnium and/or one or more rare earth elements 0.025 - 0.4 Group B: Chrome 19 - 23 Iron and/or nickel and/or cobalt residue 4. A method according to any one of the preceding claims, characterized in that the one or more rare earth elements comprise lanthanum and/or cerium. 5. A process according to claim 4, characterized in that the rare earth elements comprise lanthanum and cerium and the total content of such elements in the alloy is within the range of 0.025 to 0.07 wt.%. 6. A process according to claim 5, characterized in that the lanthanum content is within the range of 0.005 to 0.02 wt.%. 7. A process according to claim 5 or 6, characterized in that the cerium content is within the range of 0.02 to 0.05 wt.%. 8. A method according to claim 4, characterized in that the one or more rare earth elements comprise lanthanum. 9. A method according to claim 8, characterized in that the lanthanum content of the alloy is within the range of 0.06 to 0.15 wt.%. 10. A method according to any one of claims 1 to 3, characterized in that the alloy contains zirconium in a proportion of 0.1 to 0.4 wt.%. 11. A method according to claim 1, characterized in that the composition of the alloy in weight percent is as follows: Group A: Aluminum 3-8 Group B: Chrome 12 - 30 Iron and/or nickel and/or cobalt residue 12. A method according to claim 11, characterized in that the composition of the alloy in weight percent is as follows: Group A: Aluminum 4.5 - 6 Group B: Chrome 19 - 23 Iron and/or nickel and/or cobalt residue 13. A method according to any preceding claim, characterized in that the electrical resistance material (18) is heated in the atmosphere to produce a surface layer having a thickness of less than about 2 µm. 14. A method according to claim 13, characterized in that the electrically resistive material (18) is heated in the atmosphere to produce a surface layer having a thickness of less than about 1 µm. 15. A method according to claim 14, characterized in that the electrical resistance material (18) is heated in the atmosphere to produce a surface layer having a thickness of about 0.3 to 0.5 µm. 16. A method according to any one of the preceding claims, characterized in that the heating is carried out at a temperature of 900ºC to 1475ºC. 17. A method according to claim 16, characterized in that the heating is carried out at a temperature of 900°C to about 1300°C. 18. A method according to claim 16 or 17, characterized in that the heating is carried out at a temperature of at least about 1000°C. 19. A method according to claim 16, 17 or 18, characterized in that the heating is carried out at a temperature of at least about 1100°C. 20. A method according to any one of claims 16 to 19, characterized in that the alloy predominantly contains lanthanum as the active element and the heating is carried out at a temperature of at least about 1200°C. 21. A method according to any one of claims 16 to 19, characterized in that the alloy contains zirconium and the heating is carried out at a temperature of about 1300ºC. 22. A method according to any one of claims 16 to 21, characterized in that the electrical resistance material (18) is heated in the atmosphere for about 2 to about 8 minutes. 23. A method according to claim 16, characterized in that the electrical resistance material (18) is heated in the atmosphere for about 8 minutes at a temperature of about 1200°C. 24. A method according to claim 16, characterized in that the electrical resistance material (18) is heated for about 5 minutes at a temperature of about 1360ºC in the atmosphere. 25. A method according to claim 16, characterized in that the electrical resistance material (18) is heated for about 5 minutes heated to a temperature of about 1400ºC in the atmosphere. 26. A method according to claim 16, characterized in that the electrical resistance material (18) is heated for about 2 minutes at a temperature of about 1450ºC in the atmosphere. 27. A method according to claim 16, characterized in that the electrical resistance material (18) is heated for about 2 minutes at a temperature of about 1475ºC in the atmosphere. 28. A method according to any preceding claim, characterized in that heating the electrical resistance material (18) in the atmosphere oxidizes the aluminum of the Group A constituent(s) to aluminum oxide substantially in the form of alpha-alumina. 29. A method according to any one of the preceding claims, characterized in that heating of the electrical resistance material (18) in the atmosphere results in a surface layer whose permeability to oxidizing atmospheres such as air is low.